This photo represent two other ways of doing the same job. Personally I don't like either one as much as the previously mentioned methods using the Dremel bits. Why don't I like these methods as much?
1) In method #1 I drilled an over sized 1/2" hole clean through the deck and then filled it with thickened epoxy. By over drilling the entire thickness you have removed the deck skins which are a major structural component. End grain balsa has very little strength on the grain, which is where the epoxy adheres to in an over drill. Picture splitting fire wood. You split with the grain and if flakes off nicely. Epoxy adhered to the grain can do the same.
If you tried to split across the grain or pull balsa apart via the end grain it would be very, very difficult and this is why it is called "end grain" balsa and why the deck skins are laminated to the end grain not the grain of the wood. The areas highlighted in red, the skins, are thinner than they appear in this cropped photo, especially on light weight sailboats. In this example your only means of structural support are the circumferential bond between the top and bottom skins as the area in green is very, very weak comparatively speaking and would rally rely on the backing plate.
I have seen two failures of total over drills in my life. Theoretically speaking, when used with proper backing plates it should hold but if your balsa is even just moist your entire bond will only be the thin area of the two skins. It should be noted that epoxy does not adhere well to wet core this is why it is critical to seal the deck before it gets wet.
2) In method #2 I have only over drilled the top skin but left the bottom skin intact. This is slightly stronger than method #1. With the top skin gone it too is less strong than leaving most or all of the top and bottom skins intact and creating a captured plug.
3) Using either method #1 or #2 leaves you with a larger hole on the UV side of the deck. Some stanchion bases or hardware may not entirely cover the larger hole completely and epoxy does not like UV.
It's entirely up to you how you seal your decks all I can do is present the pro's and cons, as I see them. Of these two methods I prefer method #2 but I prefer the Dremel procedure far and above both methods #1 & #2..